1. Field of the Invention
The invention relates to a belt type continuously variable transmission. More particularly, the invention relates to a belt type continuously variable transmission provided with a planetary gear set which is coaxial with an output side pulley shaft of an output side pulley of the belt type continuously variable transmission, and which is also coaxial with a differential gear unit.
2. Description of the Related Art
Generally, a vehicle includes a transmission on an output side of a drive source such that a driving force transmitted from drive source such as an internal combustion engine and an electric motor, that is, torque output from the drive source is transmitted to wheels in the optimum condition based on a running state of the vehicle. There are two types of transmissions, that are, a continuously variable transmission which continuously controls a speed ratio, and a stepped transmission which controls a speed ratio in a stepwise manner. An example of a continuously variable transmission is a belt type continuously variable transmission which includes a primary pulley that is an input side pulley, a secondary pulley that is an output side pulley, and a belt. The primary pulley includes an input side pulley shaft, a movable sheave which slides on the pulley shaft in the axial direction, a fixed sheave which faces the movable sheave in the axial direction such that a V-shaped groove is formed between the fixed sheave and the movable shave, and belt holding force generating means for generating a belt holding force at a position between the movable sheave and the fixed sheave. The secondary pulley includes an output side pulley shaft, a movable sheave which slides on the pulley shaft in the axial direction, a fixed sheave which faces the movable sheave in the axial direction such that a V-shaped groove is formed between the fixed sheave and the movable shave, and belt holding force generating means for generating a belt holding force at a position between the movable sheave and the fixed sheave. The input side pulley shaft and the output side pulley shaft are provided so as to be parallel to each other. The belt is looped over the V-shaped grooves formed in the primary pulley and the secondary pulley.
In the belt type continuously variable transmission, each movable sheave is slid on the pulley shaft in the axial direction by the corresponding belt holding force generating means, whereby a width of the V-shaped groove formed in each of the primary pulley and the secondary pulley is changed. Thus, a radius of a portion where the belt and each of the primary pulley and the secondary pulley contact each other is continuously changed, and therefore a speed ratio is continuously changed. Namely, torque output from the drive source is continuously changed in the belt type continuously variable transmission.
The torque, which has been output from the drive source and then transmitted to the output side pulley shaft of the belt type continuously variable transmission, is finally transmitted to a differential gear unit that is a final reduction device. Then, the torque is transmitted from the differential gear unit to a road surface via wheels, whereby the vehicle runs forward or in reverse. Various methods of arranging the differential gear unit with respect to the secondary pulley are proposed. For example, as disclosed in Japanese Patent Application Publication No. JP(A) 2001-227615, the differential gear unit is provided so as to be coaxial with an output side pulley shaft of a secondary pulley in consideration of the space of the vehicle in which a power transmission device including a belt type continuously variable transmission and a differential gear unit are mounted. In this case, a planetary gear set that is a reduction gear unit, which decreases the rotational speed transmitted from the output side pulley shaft, is provided between the secondary pulley and the differential gear unit. Namely, the planetary gear set is provided so as to be coaxial with the output side pulley shaft of the secondary pulley.
The planetary gear set includes a sun gear, multiple pinions which are meshed with the sun gear and which are provided around the sun gear, and a ring gear which are meshed with the multiple pinions. Since helical gears as these gears are used in order to reduce gear noise, a thrust force is generated in each of the gears in the planetary gear set, when toque output from a drive source is transmitted to the planetary gear set via the output side pulley shaft of the secondary pulley.
In the above-mentioned belt type continuously variable transmission in the related art, the planetary gear set is arranged so as to be coaxial with the output side pulley shaft of the secondary pulley. Therefore, the sun gear is attached to the output side pulley shaft at an end portion on the planetary gear set side, or the sun gear is formed integrally with an input shaft. Therefore, due to the thrust force generated in the sun gear, the output side pulley shaft moves in the axial direction, and therefore the secondary pulley moves in the axial direction. At this time, the primary pulley is also dragged and moved by a belt, which is looped over the primary pulley and the secondary pulley, in a direction in which the secondary pulley moves.
The thrust force is changed according to a change in the torque output from the drive source. Therefore, if the torque output from the drive source changes abruptly, a positional displacement of the output side pulley shaft with respect to the input side pulley shaft in the axial direction may occur instantaneously. Namely, a displacement of a relative position between the primary pulley and the secondary pulley in the axial direction may occur instantaneously.